The immunoglobulin binding domain B1 of streptococcal protein G (herein sometimes referred to as GB1), as illustrated in Park, et al. Biochemistry, 36:14277-14283 (1997), is a small protein with a well developed hydrophobic core, that folds into a 4-stranded beta sheet with a flanking alpha helix. This protein contains no disulfides or free cysteines, no prolines or metal binding motifs. The GB1 is highly stable and soluble and is one of the most extensively used model systems in the area of protein folding and design.
In addition, the GB1 contains on its' surface binding sites for the C-terminal fragment of the heavy chain of immunoglobulin G (IgGFc). Proteins containing this domain can be purified by immobilized Fc, eluted from the affinity column by acid or proteolytically clipped off and detected by existing reagents.
It has been reported by Hammarstrom, et al. Protein Sci., 11:313-321 (2002) comparing soluble expression levels of protein fusions with this B1 domain with fusions using MBP, NusA, ZZ, GST, Thioredoxin and His6 and 26 different proteins, that GB1 gave a higher percentage of soluble expressed proteins in E. coli. It has also been reported by Zhou, et al. Journal of Biomolecular NMR, 20:11-14 (2001) that when used as an epitope tag for fusion with E. coli expressed recombinant proteins, this domain does not need to be removed to obtain NMR structure. Finally, Cheng & Patel, Biochem. Biophys. Res. Commun., 30:401-405 (2004) concluded that expression of GB1 containing proteins in E. coli increases expression levels.
In a majority of instances, because of protein folding and solubility issues, many recombinant proteins cannot be expressed in prokaryotic organisms. Therefore, mammalian cells are the expression system of choice for recombinant proteins needing extensive post-translation modifications. However, even with mammalian expression systems, low expression and low activity of the expressed recombinant proteins using small epitope tags, such as 6× Histidine and Strep-tag II, have negatively impacted large scale purification for crystallography and HIS. Data obtained from the expression of Endothelial Lipase indicated that with either no tag, with a 6× Histidine tag, or with a 6×Histidine/Strep-tag II, expression was very low, at levels >1 mg/liter.
It has been demonstrated that use of the IgGFc region as a tag leads to very high levels of protein expression. However, this tag can be detrimental to genes requiring a particular orientation such as a reverse dimer. They block proper folding and, thus, lead to loss of activity.
Thus, there remains a need for expression or overexpression of selected polypeptides in mammalian cells. It was totally unexpected to discover, in the instant invention, a dramatic increase in expression levels in mammalian cells using a GB1 tag over the traditionally used tags, and to see full activity in all of the proteins examined.